Hello Webio,
Thanks for the inquiry. I have observed the log file to exceed the default values. I believe the limit is set to 1MB. Please look into either copying the two Clam files [freshclam.conf | clamd.conf] out of the following path: [C:\Program Files (x86)\SmarterTools\SmarterMail\Service\Clam\etc] and then deleting the files and they will be rebuilt.
You can opt to modify the Clamd.conf file to increase the size limit. Since the ClamAv is a third party application you may want to refer to the documentation on ClamAv's website for more specifics.
Thanks.